Abstract This project is a focused on the continuation of the epidemiological research capability of the Centers for Autism and Developmental Disabilities Epidemiology (CADDRE) network with implementation of Phase 3 of the Study to Explore Early Development. The ongoing objectives of the SEED study are to characterize the autism behavioral phenotype and associated developmental, medical and behavioral conditions of autism. and to understand the genetic and environmental risk factors of Autism. The aims of this application are as follows: 1. Implement a third Phase of the SEED study preserving the core study design with enhancements to improve recruitment and a reduction in the data collection protocol in a new Midwestern SEED site. The following aspects of the core study design will be preserved in Phase 3: a) a case-control study design with multiple- source ascertainment of an ASD case group, a non- ASD developmental disability (DD) comparison group and a population-based (POP) comparison group, b) clinical confirmation of developmental status of all participants using a standardized assessment protocol, and c) collection of clinical, genetic and environmental data. Recruitment enhancements will include active follow-up procedures for invited participants. Data collection will be streamlined, compared to Phase 2, primarily by eliminating pediatric and neonatal medical record abstraction for all three participant groups and curtailing data collected for the DD comparison group. 2. Further increase the total SEED pooled sample size to enhance SEED's analytic potential to investigate rare exposures and ASD subtypes by achieving complete data collection on 125 ASD children, 125 DD children and 125 POP children within our site. Reaching these targets at all anticipated Phase 3 sites will increase the total SEED ASD sample to approximately 2000 children which will enhance the examination of rare exposures and modest associations potentially relevant to the etiology of ASD.